Reporting Datum Target Dimensions

[Vindicit]

I have a drawing with datum targets thats similar to what I have attached with 3 bolt holes inside the profile. I need to do a first article inspection on this part and I dont know how I should inspect and report these dimensions for the datum targets. I am using an Optical CMM and I just use the bolt holes as a datum alignment so I dont need to use the profile. I can use the 2 outside radii as datums but the 45 degree and Basic .212 do nothing for me. How would I go about inspecting this part with these datum targets and how would I report them? Or do I?

 

[pmarc]

Basic dimensions should not be inspected and reported. In this case they are telling you how the datum targets look like and how they are located relative to each other.

 

[Vindicit]

I understand that basic dimensions are usually not reported but on First Article reports for Aerospace Im being required to report everything even if its not a rejectable requirement. So knowing that, are datum targets like this intended to be inspected at all? Would I use this information for anything if I have a CMM?

 

[Belanger]

It's true that basic dims don't get reported. And as pmarc wrote, the same is true for datum targets. This is because there is nothing to measure with datum targets; they merely tell you how to "hold" the part in your CMM for some other tolerance that will reference B and C (or maybe B-C).

Think of a regular datum, symbolized by a triangle. When you see a triangle symbol for datum A, you don't report anything -- it's just telling you which surface to zero out on. The same thing applies here! The only difference is that with datum targets they're getting a little more picky about which portions of a surface you probe in order to establish a datum.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems